Treatment of coatings



Patented July 23, 1935 UNITED STATES PATENT OFFICE TREATMENT OF COATINGSNo Drawing. Application May 26, 1932, Serial No. 613,793

20 Claims.

The invention relates to methods of treating oxide coatings, composed insubstantial part of aluminum oxide, to alter or modify the propertiesand characteristics of the coatings. Particularly is the inventionconcerned with the treatment of oxide coatings formed on aluminum andaluminum base alloy surfaces, herein collectively referred to asaluminum surfaces.

By several known processes aluminum surfaces may be provided with Whatis generally termed an oxide coating. This coating is substantiallycomposed of aluminum oxide. It may contain, because of the process used,other components which essentially alter its specific characteristics.

Such oxide coatings have, as general properties, a good resistance tocorrosion, a good resistance to abrasion, and an ability, in variousdegrees, to take organic dyes or inorganic colors to form a coloredsurface. The oxide coatings may be more or less permeable and adsorb orabsorb moisture or liquids. Quite often, also, the coating forms anexcellent insulation against the passage of electric current.

The oxide coatings may be formed on aluminum surfaces by variousmethods. The aluminum may be made an anode in an electrolytic cellcontaining an electrolyte such as sulfuric acid, chromic acid, organicacids, acid salts, etc. When external electrical energy is impressedupon 39 the ce l an oxide coating is formed on this anode. In anothermethod the aluminum is immersed in a suitable solution, generallyalkaline, and the oxide coating is formed by chemical reaction withoutthe use of external electrical energy.

35 Other processes are known and used to produce artificially theseoxide coatings. In all cases the oxide coating to which this inventionrelates is produced by artificial means, The natural film of oxideoccurring on all aluminum surfaces is not herein referred to as an oxidecoating.

The oxide-coated aluminum surface may then be immersed in organic dyes,or in solutions of dyes, to produce characteristic colors.

45 The object of the present invention is to treat oxide coatings formedon aluminum surfaces to produce certain results, among which may benumbered: a decrease in the permeability of the oxide coating; amodification of the adsorptive 50 or absorptive capacity of the oxidecoating for coloring agents; increase in corrosion resistance of thecoating; increase of the ability of the colored oxide coating to retaincolor under the action of solvents. Another object of the inven- 55 tionis to treat colored oxide coatings after the coloring operation touniformly fix said color in and on the coating.

The objects are obtained in lesser or greater degree depending upon theparticular coating treated and the manner in which the treatments 5hereinafter described are used, but in every case a new or improvedresult is obtained which greatly enhances the usefulness of theoxidecoated aluminum and the breadth of its commercial application. 10

The invention is predicated upon the discovery that by the treatment ofoxide-coated aluminum in hot or boiling solutions of certain salts theproperties of the coating are considerably modified and new andadvantageous properties are 15 developed. The solutions to which thisinvention refers are solutions containing a salt of a weak metallic basewhich salt is capable of hydrolyzing in solution to form a substantiallyinsoluble compound of the metal in the oxide coat- 20 ing. The hydrogenion concentration of the solution should be such that the hydrolysis inhot or boiling solution promotes precipitation of the metal compound andthis condition is usually evidenced by the appearance of turbidity in 25the solution. Among the salts which will form such solutions (withproper hydrogen ion adjustment) are metallic acetates, such as nickel,cobalt, cadmium, zinc, barium, copper, aluminum or lead acetates, andmetallic sulfates, oxalates, chlorides, nitrates, citrates, tartratesand fluorides, but there are other salts of similar chemicalcharacteristics which will produce the desired results.

While the invention broadly contemplates the use of such solutions, inits specific and preferred practice certain salts are preferable toothers. For instance, because of the difiiculty involved in maintainingproper hydrogen ion concentration, solutions of the salts of weak acidsand weak metallic bases are preferable in commercial operation, providedthat they have the properties of the general class above stated, i. e.,especially the property of hydrolyzing in hot or boiling solutions toform a precipitate in the oxide coating. Likewise in specificapplications of the practice of the invention, it is often highlydesirable that the salt solution be one which does not color the coatingand/or it be one which does not leave in the coating a poisonous ortoxic element. Also it is desirable from many commercial viewpoints thatthe salt solution be one that does not materially attack or dissolve theoxide coating or the aluminum. Such preferred properties 01' the saltsolution are, however, desirable only in certain applications of theinvention, and the broad class of salt solutions herein defined are, perse, commercially useful.

The solutions used in the practice of the invention are prepared bydissolving a salt of the class described in water and then, ifnecessary, adjusting the hydrogen ion concentration to the point wherehydrolysis in hot or boiling solutions, i. e., in solutions heated above40 centigrade, will promote precipitation in the oxide coating. Thehydrolysis, under these conditions, may be such that suspension ofcolloidal dimensions is formed in the solution and no precipitate isevident. But if, on heating, a precipitate is evident, the hydrogen ionconcentration is so adjusted that a substantial amount of the metallicbase is not precipitated in fiocculent form and does not settle from thesolution. In solutions where, on hydrolysis, precipitation is evident,it is desirable that the hydrolys's does not substantially proceed pastthe point of turbidity. The term turbidity as used herein and in theappended claims denotes not only a turbid but likewise an opalescent orcloudy solution. In the practice of my invention with preferredsolutions such turbidity is customarily observed. The proper hydrogenion concentration is readily predetermined by simple experiment, but itis preferred and usually advisable to use solutions having a hydrogenion concentration, as measured at 25 centigrade, of between a pH ofabout 4.5 and a pH of about 7.5. When the solution has pH values widelyvariant from this range, the proper solution conditions are not usuallyobtained and, moreover, the oxidecoated aluminum will be attacked. Inthe case of each solution, however, the proper hydrogen ionconcentration is readily and simply determined by experiment, such as byimmersing several pieces of oxide-coated aluminum in solutions ofvarying hydrogen ion concentration and observing the results obtainedthereby.

When the salt is one of a weak metallic base and a strong acid it isvery desirable to add to the solution a buffer such as boric acid,borates, acetates, or other well known buffering agents, in order thatthe proper hydrogen ion concentration may be readily obtained andmaintained, the buffering agent tending to reduce the sensitivity of theadjustment. Likewise when, in preferred practice, a salt of a weak baseand a weak acid is used, the employment of a buffering agent may bedesirable in the interests of maintaining the predetermined hydrogen ionconcentration desired. For instance, excellent results have beenobtained by adding boric acid to solutions of nickel or cobalt acetate.

The salt solution may contain large or small amounts of a salt of thegeneral class described. While amounts of 0.1 per cent by weight havebeen found eflicient, and solutions containing the salt to the maximumsolubility thereof are useful, it is preferable in commercial solutions,when solubility relations permit, to use about 1 or 2 per cent of thesalt in solution. From time to time the solution may have to be renewedas it becomes exhausted or inactive by use.

The oxide-coated aluminum is immersed in the hot or boiling saltsolution for a time sumcient to effect the result desired. The time oftreatment varies with the particular coating and with the Particulareffect desired. It is usually between about 2 and 30 minutes.

In order to describe the effects obtained, several of the uses to whichthe methods of the invention may be placed will be specificallymentioned.

Quite often the oxide coating is placed upon aluminum utensils such astrays, ice-freezing receptacles and the like. In such cases the coatingis not colored since the natural oxide-coated aluminum finish isdesired. Such trays and receptacles are often in cont/act with coloredliquids, colloidal or otherwise, and such liquids, if adsorbed by theoxide coating, stain the coating. By treating the coated articles insalt solutions in accordance with this invention, the coating is madesubstantially non-porous and impermeable, thus eliminating or materiallydecreasing the propensity of the coating to stain. When the methods ofthe invention are thus used, it is desirable to employ salt solutionswhich do not, themselves, color the oxide coating or act as a mordantfor coloring agents.

Or, again, the aluminum article may have been provided with a coloredoxide coating or with a coating containing in its pores a substance suchas sodium silicate, which will increase the corrosion resistance of thecoating. Such a coated aluminum article may, in its use, be subject todetrimental external agencies such as moisture, general weatherconditions, solvents, etc., which will tend to remove the coloring orother materials from the pores of the coating. By treating the coatedarticle in accordance with this invention, in salt solutions of theclass described, the coating is made impermeable or substantially so;and, at the same time, the useful substance, coloring matter orotherwise, is more or less permanently bound in the pores of the oxidecoating and protected against the action of solvents, etc.

When an oxide-coated aluminum article is dipped into a solution oforganic dye, as is commercial practice, most of the dye which remains onthe article, after removal from the dye solution and rinsing, isadsorbed in the oxide coating, but a certain amount of the dye lying onor adjacent the coating surface is not so firmly bound to the coating.Consequently this dye tends to rub ofi in use. To cure this it has beenthe practice to bufi the colored coating, but buffing is often expensiveand, in the case of small articles, such as buttons and pins,practically impracticable. By treating the article according to methodsof the present invention, the dye on or adjacent the surface issubstantially fixed on said surface so that crocking or smudging areminimized.

While any of the general class of salt solutions herein defined willgive the results described in greater or less degree, there are, as hasherein been stated, certain salts which are preferred to others. Amongsuch salts are found those which are used in the preferred commercialpractice of my invention; viz., nickel acetate, cobalt acetate, nickelsulfate and cobalt sulfate; of these named salts the acetates aregenerally preferred. Buffering agents are useful in solutionsof eitherthe sulfate or the acetate.

I claim:

1. The method of depositing in an Oldfiecoated aluminum surface aninsoluble compound by hydrolyzing a salt of a weak metallic base insolution, and immersing said oxide-coated aluminum in said solution.

2. The method of depositing in an oxide-coated aluminum surface aninsoluble compound formed by hydrolysis of a salt of a weak metallicbase, by immersing said oxide-coated aluminum in a solution of a salt ofsaid weak metallic base, and maintaining the temperature of saidsolution above about 40 centigrade.

3. The method of depositing in an oxide-coated aluminum surface aninsoluble compound formed by hydrolysis of a salt of a weak metallicbase, by immersing said oxide-coated aluminum in a solution of a salt ofsaid weak metallic base having a pH of 4.5 to 7.5.

4. The method of depositing in an oxidecoated aluminum surface aninsoluble compound formed by hydrolysis of a salt of a weak metallicbase, by immersing said oxide-coated aluminum in a solution of a, saltof said weak metallic base having a pH of 4.5 to 7.5, and maintainingthe temperature of said solution above about 40 centigrade.

5. The method of depositing in an oxidecoated aluminum surface aninsoluble compound formed by hydrolysis of a salt of a weak metallicbase, by immersing the said oxide-coated aluminum in a solution of asalt of the weak metallic base and a weak acid and maintaining thetemperature of the solution above about 40 centigrade.

6. The method of treating oxide-coated aluminum surfaces by immersingthe same in a solution of a salt of a weak metallic base in whichhydrolysis proceeds to the point of turbidity and which contains nosubstantial amount of flocculent precipitate.

7. The method of treating oxide-coated alumi-= num surfaces by immersingthe same in a solution of a salt of a weak metallic base and a weak acidin which hydrolysis proceeds to the point of turbidity and whichcontains no substantial amount of fiocculent precipitate.

8. The method of depositing in an oxide-coated aluminum surface aninsoluble compound formed by hydrolysis of a salt of a weak metallicbase, by immersing said oxide-coated aluminum in a solution of a. saltof a weak metallic base which contains also a buffering agent, andmaintaining the temperature of said solution above about 40 centigrade.

9. The method of depositing in an oxide-coated aluminum surface aninsoluble compound formed by hydrolysis of a salt of a weak metallicbase, by immersing said oxide-coated aluminum in a solution of a salt ofa weak metallic base and a strong acid which contains also a bufferingagent, and maintaining the temperature of said solution above about 40centigrade.

10. The method of depositing in an oxide-coated aluminum surface aninsoluble compound formed by hydrolysis of a nickel salt, by immersingsaid oxide-coated aluminum in a solution of said nickel salt having a pHof 4.5 to 7.5.

11. The method of depositing in an oxidecoated aluminum surface aninsoluble compound formed by hydrolysis of a cobalt salt, by immersingsaid oxide-coated aluminum in a solution of said cobalt salt having a pHof 4.5 to '7 .5.

12. The method of treating oxide-coated aluminum surfaces by immersingthe same in asolution of a nickel acetate; the temperature of thesolution being above 40 centigrade.

13. The method of treating oxide-coated alu- 4 minum surfaces byimmersing the same in a solution of a nickel acetate; the solutioncontaining also a buffering agent and having a temperature above 40centigrade.

14. The method of treating oxide-coated aluminum surfaces by immersingthe same in a solution of nickel sulfate; the temperature of thesolution being above 40 centigrade.

15. The method of treating oxide-coated aluminum surfaces by immersingthe same in a solution of nickel sulfate; the solution containing also abuffering agent and having a temperature above 40 centigrade.

16. The method of treating oxide-coated aluminum surfaces by immersingthe same in a solution of cobalt acetate; the temperature of thesolution being above 40 centigrade. v

17. The method of treating oxide-coated aluminum surfaces by immersingthe same in a solution of cobalt acetate; the solution containing also abuffering agent and having a temperature above 40-" centigrade.

18. The method of treating oxide-coated aluminum surfaces by immersingthe same in a solution of cobalt sulfate; the temperature of thesolution being above 40 centigrade.

19. The method of treating oxide-coated aluminum surfaces by immersingthe same in a so lution of cobalt sulfate; the solution containing alsoa buffering agent and having a temperature above 40 centigrade.

20. The method of treating oxide-coated aluminum surfaces by immersingthe same in a solution of cobalt sulfate; the solution containing also abuffering agent.

MARTIN TOSTERUD.

